Emulsion polymerisation provides one of the most effective means of preparing an aqueous dispersion of polymer particles. Accordingly, this polymerisation technique has been extensively adopted by industry to manufacture aqueous dispersions suited for use in products such as paints, adhesives, fillers, primers and sealants.
The conventional emulsion polymerisation system initially comprises water, monomer, surfactant and initiator. The emulsion polymerisation process generally commences by dispersing monomer (organic phase) in the water (aqueous phase), with the aid of the surfactant, to provide an emulsion. The initiator, which is usually dissolved in the continuous aqueous phase, provides a source of the free radicals that initiate polymerisation. The dispersed organic phase provides monomer to the propagating polymer chains which in turn form small polymer particles. During formation and in a final form, the polymer particles are stabilized from coalescence by the surfactant. The polymerisation process therefore provides as a product an aqueous dispersion of polymer particles.
Although very useful in providing aqueous dispersions of polymer particles for commercial uses, current emulsion polymerisation technology presents some inherent problems. For example, when a dispersion, or product prepared from a dispersion, is applied to a surface and dries to form a film, as with a paint, free surfactant in the dispersion can tend to migrate to the surface and localise in pockets, thereby adversely affecting the surface properties of the film, particularly in the area of water sensitivity. Also, polymerisation is typically achieved by a classical free radical polymerisation process which has a limited capacity to effectively control both molecular weight and architecture of the resulting polymer, and no ability to produce block copolymers.
One approach to restricting migration of the surfactants has been to use amphiphilic compounds that have an unsaturated hydrophobic tail, so called “surfmers”. During polymerisation, the surfmers stabilize monomer, allowing the polymer particles to grow in a conventional manner. The unsaturated hydrophobic tail, which becomes buried within a growing polymer particle, can react with a propagating chain to effectively anchor the surfmer to the particle. However, the use of such a technique provides little ability to control the architecture of the resultant polymer particles.
Options for modifying the radical chemistry of the polymerisation reaction have been quite limited. However, recent developments in free radical chemistry have to some extent broadened the scope of chemistry available for potential adaptation to emulsion polymerisation. In particular, so called controlled/living radical polymerisation techniques such as nitroxide mediated radical polymerisation (NMRP), atom transfer living polymerisation (ATRP), degenerative transfer techniques best exemplified by reversible addition-fragmentation chain transfer (RAFT) have been investigated (Macromolecules 2001, 34, 5885-5896).
The RAFT process, as described in International Patent publication WO 98/01478, is a radical polymerisation technique that enables polymers to be prepared having a well defined molecular architecture and a low polydispersity. The technique employs a chain transfer agent (CTA or RAFT agent) of the general formula (1):
which has been proposed to react with a propagating radical (Pn*) in accordance with Scheme 1.

The effectiveness of the chain transfer agent (1) is believed to depend on a complex array of rate constants. In particular, the formation of polymer according to scheme 1 is believed to be reliant upon equilibria that require high rate constants for the addition of propagating radicals to agent (1) and the fragmentation of intermediate radicals (2) and (3), relative to the rate constant for propagation.
The rate constants associated with RAFT polymerisation are influenced by a complex interplay between stability, steric and polarity effects in the substrate, the radicals and the products formed. The polymerisation of specific monomers and combinations of monomers will introduce different factors and structural preferences for the reagent 1. The interplay of factors for a particular system have been largely rationalised on the basis of the results obtained. A clear definition of all factors that influence polymerisation for any particular system has not yet been determined.
While RAFT technology provides for the preparation of block copolymers using free radical polymerisation, and can provide means for superior control over many polymerisation processes, difficulties have been encountered in using the technology in emulsion, miniemulsion, suspension polymerisation processes and the like. Successful adaptation of RAFT chemistry to an emulsion polymerisation requires the polymerisation conditions to be such that the polymerisation process can proceed under RAFT control. Furthermore, in order to maintain control over polydispersity and molecular weight, the RAFT agent must be located at the reaction loci (nucleated particles) at the start of the polymerisation and be homogeneously distributed amongst all particles. To achieve these conditions, a RAFT agent should be sufficiently water-soluble so as to diffuse from a monomer droplet to a nucleated polymer particle in a time frame that is much faster than the duration of the polymerisation and which is also much faster than the nucleation period. Alternatively, a water miscible co-solvent could be used to aid the migration of the RAFT agent. Such requirements may be met by “fine tuning” the reaction system, but this is difficult to achieve in practice.
Alternative modes of performing an emulsion polymerisation, such as miniemulsion or seed emulsion techniques, have recently been shown to alleviate problems associated with the diffusion of RAFT agents. In both cases, the RAFT agent can be directly and uniformly introduced to the polymerisation loci prior to starting the reaction, thereby satisfying the aforementioned requirements. Such techniques have been shown to provide superior control over the polymerisation process compared with classical free radical polymerisation. However, both techniques employ conventional surfactants and dispersions prepared thereby are subject to the aforementioned surfactant migration problems. Furthermore, both techniques require co-surfactant stabilizers and other additives which introduce unwanted components into the polymerising mixture and compromise the properties of the finished product to a point where the potential benefits of the RAFT process cannot be demonstrated.